The present invention relates to coherent imaging systems using vibratory energy, such as ultrasonic or electromagnetic waves, and, more particularly, to a novel method for forming the vibratory (ultrasonic) beam, including beam direction (steering), focussing and apodization functions, totally by digital (rather than analog) signal processing of the vibratory (ultrasonic) signal.
It is now well known that ultrasonic imaging systems can provide many benefits in the various analytic arts, such as medicine and the like. A particularly beneficial form of ultrasonic imaging utilizes a phased array sector scanner (PASS) to sweep a formed beam with the greatest speed and accuracy. Originally, analog signal processing techniques were utilized to perform a coherent sum of the various signals received across the plurality N of elements in the front end of the PASS array. That is, it is well known that the accuracy of the beam formation, and particularly the pointing direction thereof, is directly related to the accuracy of the phase relationship, or time delays, between the various elements of the PASS array. It has been shown that a phase accuracy of about one part in 32 is necessary to form ultrasonic beams with enough accuracy for medical imaging applications. Therefore, each of the time delays in the PASS array must be adjustable with accuracy at least as small one-thirty second (1/32) of the time interval required for a single cycle of the imaging system fundamental frequency. For example, with an imaging system fundamental frequency of about 4.5 MHz., a time delay accuracy of about 7 nanoseconds is required. Because of this requirement, earlier systems have been somewhat replaced by systems using baseband signal processing, such as described and claimed in U.S. Pat. No. 4,155,260, issued in 1979, and application Ser. No. 794,095, filed Oct. 31, 1985 now U.S. Pat. No. 4,669,314, both assigned to the assignee of the present application and incorporated herein by reference in their entireties. The baseband processing system is such that phase accuracy and time delay accuracy are decoupled from one another, to dramatically reduce the requirements on the circuits used for beam formation. That is, the phase characteristics of the baseband demodulators are controlled so that the phase relationships between the RF signals received at the array elements are preserved after transfer to the baseband frequencies. Therefore, the demodulated signals can be coherently summed, which results in a dramatic reduction in the accuracy necessary for the time delays, which are now at the baseband (rather than RF) frequencies. However, even with baseband frequency processing, a PASS array front end is: relatively inflexible; sensitive to minor variations in the properties of analog circuits; and is relatively costly (as 2N individual analog demodulation circuits and 2N individual and complete time delay sections are required for a N channel array).
A fully digital PASS front end will allow real-time beam formation to be carried out in an accurate, flexible and cost-effective manner. While fully digital systems, such as in U.S. Pat. No. 4,324,257 and the like, were first proposed in the 1970s to attempt to overcome some of the inflexibility of analog processing, the fully digital systems described, to date, in the literature have not yet produced beams acceptable for medical imaging applications. The major problem appears to be that the time delay accuracy of such systems, being determined by the sampling rate of the analog-to-digital converter (ADC) means utilized therein, have typically been an order of magnitude less than the level of accuracy needed for medical applications, where the beam is formed of energy in the 2-5 MHz. range. That is, the ADC means in such systems have sample capabilities of between about 10 MHz. and about 20 MHz., so that resulting time delay accuracies of only between about 100 nsec. and about 150 nsec. can be obtained, rather than the desired accuracies of between about 6 nsec. and about 15 nsec.
As many other forms of vibratory energy can be used, such as coherent electromagnetic energy in ladar and radar imaging systems, as well as other types of acoustic energy systems (sonar and the like), it is desirable to provide beam forming methods and apparatus useful in any system for obtaining an image of an object by reflection of an impingent beam of vibratory energy.